This invention relates to the elimination of backlash between gears, and, more particularly, to the elimination of backlash between the meshed gearing in an electric power steering system.
In the prior art, rack and pinion steering devices for motor vehicles are well known and generally comprise a motor driven pinion that rotates a nut through which a steering rod translates. The rack forms a part of the steering rod to which steerable wheels of the motor vehicle are attached. In these prior art steering devices, backlash, or clearance around a tooth of the driver gear as it fills the space between two teeth of the driven gear, is necessary in order to permit relative motion between the two gears. In a gear system with no backlash, the meshing of the teeth between the gears will be so tight that, absence any deflection of the teeth, the gears will bind and cause the system to jam. However, in a gear system with any degree of backlash, an audible noise known as xe2x80x9clashxe2x80x9d is produced by the contact of the teeth of one gear with the teeth of another gear. While lash is not indicative of a defect in the gearing system, it often proves to be an annoyance and it is therefore desired to be minimized as much as practicable.
Various attempts to de-lash gear systems are also well known in the prior art. In a rack and pinion system in which the gear sets are of a parallel axis structure, shims may be used to bias the pinion against the rack. The use of shims, however, requires a significant amount of trial and error technique in order to find the correct amount of backlash needed to allow the system to function properly. Furthermore, the installation of shims within a gearing system requires periodic maintenance to correct any slight changes in the system that occur over time due to vibration and normal use.
Coil springs are also often used to load rack and pinion systems and to urge a rack shaft resiliently against a pinion shaft to adjust the backlash and eliminate any lost motion between the two gears. In a variation of this system, the pinion may be resiliently urged against the rack. In either system utilizing resilient coil springs, the loading of either the pinion or the rack in one direction takes up the clearance between the teeth of the rack and the teeth of the pinion. However, because in the driving of a gear there is generally only single flank contact between the teeth of the pinion and the teeth of the rack, the driving of the pinion in the opposite direction will usually result in lost motion and cause backlash.
Still another system used to drive the rack is a belt transmission system. In such a system, the pinion drives a pulley, which is connected to the ball nut with a belt. Rotation of the pulley necessitates the movement of the belt, which drives the ball nut, thereby causing the ball screw, which is integrally formed with the rack, to translate the ball nut. In such a system, backlash is initially eliminated due to the absence of gears; however, over time the belt stretches and the slack in the belt introduces a certain amount of backlash to the system. Because the belt stretches, corrective maintenance is required on a more frequent basis. Furthermore, replacement of worn or broken belts is often a difficult task that requires at least partial disassembly of the rack and pinion system.
A steering gear system is described for an electric power steering system in which a directional change in the driving of a pinion will limit the amount of backlash experienced by the rack and pinion gears or eliminate the backlash altogether. The system will further limit the amount of backlash between a rotor shaft of a drive motor and the pinion when the pinion or the rotor shaft are moved angularly relative to each other. The steering gear system utilizes a gearing mechanism in which a pinion and a sub-pinion are urged apart by a resilient member such that the threads on each are always in contact with the flanks of the teeth on the rack. The steering gear also utilizes at least one spring placed between the rotor shaft and the pinion to enable rotor shaft to xe2x80x9cfloatxe2x80x9d within the pinion in order to compensate for angular motion of the rotor shaft relative to the pinion.
The steering gear system includes a motor, a gearing mechanism driven by the motor, a drive gear in mechanical communication with the gearing mechanism, a ball nut driven by the drive gear, and a rack laterally positioned through the ball nut and configured to translate the ball nut upon rotation thereof. The gearing mechanism includes a pinion and a sub-pinion in mechanical communication with each other held in union but urged apart by a resilient member disposed therebetween and at least one spring positioned between an inner surface of the pinion and an outer surface of the rotor shaft. Helical gear threads disposed on the pinion and sub-pinion are configured to engage corresponding threads on the drive gear. The resilient member forces the flanks of the threads on the pinion and the sub-pinion to remain in contact with corresponding threads on the drive gear regardless of the direction of rotation of the pinion/sub-pinion assembly.
The pinion comprises a body portion and a protrusion, and the sub-pinion comprises a body portion and a hole configured to receive the protrusion of the pinion. Splines are disposed on the outside surface of the protrusion and the inside surface of the hole of the sub-pinion and are inter-engageable to prevent the axial rotation of the pinion relative to the sub-pinion. A retaining ring is positioned over the part of the protrusion that extends through the body portion of the sub-pinion to hold the pinion and the sub-pinion in union. The spring between the pinion and the rotor shaft, which may be frictionally retained therebetween or recessed into either the pinion or the rotor shaft, is a ring-shaped member-configured to deform in a radial direction when flexed in an axial direction and is configured to urge the pinion away from the outer surface of the rotor shaft when the spring is received over the rotor shaft. The resilient member is configured to urge the pinion and the sub-pinion apart as each are held in union and is positioned in a channel either in the body of the sub-pinion concentrically about the hole or in the body of the pinion concentrically about the protrusion. The entire gearing mechanism may be enclosed in a housing installable in the engine compartment of the motor vehicle.
The above system eliminates the backlash associated with the directional changes of pinions of the prior art as well as backlash caused by angular movement of the rotor shaft relative to the pinion. The elimination of backlash improves the performance, quality, and efficiency of the gearing system by maintaining contact between the teeth of the pinion and the teeth of the rack while eliminating the intermittent contact therebetween, which can contribute to wear of the teeth of the system. The elimination of intermittent contact between the teeth of the gears further serves to limit the amount of noise generated by the system.